We propose to exploit the site specific bacterial restriction endonucleases to study the structure and function of SV40 tumor virus genome. The restriction endonucleases will be used to generate genome fragments which can be separated by polyacrylamide gel electrophoresis. The contiguous relationship of the fragments can be determined and the fragments systematically reconstructed into intact genomes (recombinants) or mutant genomes with added or deleted pieces of DNA. The recombinants and mutants will be used to investigate the role(s) of the SV40 genes in the lytic and transformation cycles of the virus with the ultimate goal of elucidating those function(s) of the genome responsible for transformation of the host cell.